Reduction equipment has been known for a number of years and the sizes and applications of such devices vary widely. In the yard and garden equipment industry, reduction mills are becoming more commonplace as states and municipalities mandate the composting of yard and garden waste, or as operators of composting sites find that their operations can be run more efficiently if waste such as branches, fallen trees, and the like are comminuted before the material is put into windrows or piles. In the refuse industry reduction mills are also becoming commonplace as the Federal and State governments mandate strict requirements for landfills. The smaller pieces resulting from such operations biodegrade more quickly under suitable moisture and oxygen conditions and the volume required for the ultimate disposal of the material is also reduced.
Such machines have included a generally rectangular collection hopper which can be loaded by front end loaders and the like with debris to be comminuted. The floor of the bin is a first endless conveyor adapted to move the debris from a rear portion to the opposite end of the machine. Prior machines have also included an upper conveyor, inclined at an acute angle with respect to the floor conveyor, or an upper feed roll adapted to assist in moving material toward the nip formed between the two conveyors. A rotating hammer mill has been located at the outlet of the nip to receive material being moved by the conveyors. The hammer mill includes a rotating hammer roll having plurality of hammer knife elements which pass in close proximity to a stationary cutting surface, all as is well known in the comminuting art for dividing the material into fine pieces which are discharged at the rear of the machine. Various modifications which are not relevant to the present invention include providing screens on the rear of the hammer mill to cause particles to stay in the shredding section for a longer period of time so that the average particle size can be reduced, and various devices for directing the discharge to a desired outlet location, which could be a windrow, a pile or the like.
In such prior equipment, one frequently encountered problem has been the introduction of unshredable material into the machines when large bunches of the debris are being forced by the two conveyors toward the hammer roll. Unshredable material may include stones, concrete or metal hidden in the debris. Due to a lack of suitable equipment to detect such unshredable material in the prior art machines, unshredable material has resulted in serious and costly damage to the equipment. Removal of the unshredable material from the hammer mill area necessarily requires the equipment to be completely shut down so that the conveyor and hammer may be cleaned out manually. Only after the debris has been cleaned from the machine can the damage be assessed and repaired. Such operations result in reduced efficiency and substantially increased operating costs for the equipment, and a system which would overcome this problem would represent a substantial advance in this technology.
Attempts at designing the equipment to withstand introduction of the unshredable material have met with minimal success. These devices generally center around screens or spring loaded by-pass gates.
For example, U.S. Pat. No. 3,082,963, teaches a hammer grinder. The device includes a vertical supply passage above the hammer roll and a semi-cylindrical grid below the hammer roll. A casing is located adjacent to the supply passage for collecting unshredable material. The unshredable material is removed from the hammer grinder through an opening in the lower part of the casing.
U.S. Pat. No. 3,540,665, teaches a scrap breaking device. The device includes a supply passage for the scrap positioned above the hammer roll and a semi-cylindrical grid below the hammer roll which forms a partition between the hammer roll and the a discharge passage. The scrap first enters the supply passage via a conveyor and falls down into the hammer roll area. The objects which are not broken are thrown upward to a grid positioned above the conveyor. Objects remaining on the grid can be removed manually after the hammer roll is shut down.
U.S. Pat. No. 4,378,094, teaches a material reducing mill. Material is delivered to the mill via a conveyor which allows the material to fall onto the rotating hammer roll. The device is also provided with a by-pass gate positioned adjacent to the fall of material. The by-pass gate is manually operable to direct unshredable material away from the hammer roll. When an operator hears a unshredable object strike the hammer roll a lever is actuated to move a gate into an open position, thereby allowing the material in the area to be by-passed around the hammer roll assembly. The result is that the chute formed thereby will direct a quantity of material, including the unshredable material, into the by-pass passage. Concurrently with the operation of the by-pass gate, the drive for the conveyor is reversed so that the by-passed material can be directed into a container.
U.S. Pat. No. 4,449,673, teaches a reduction mill having a rotating hammer roll and a hydraulically displacable grate and by-pass door assembly. The grate assembly being pivotally displacable by power means, as a single unit, from the hammer roll portion to achieve ready access to the grate assembly for reversal or replacement of worn sections. The by-pass door is pivotally displacable to a first position which permits quick, safe and efficient removal of unshredable materials from the product stream of the reduction mill without stopping hammer roll rotation. The pivotal displacement of the by-pass door is selectively powered by the same power means or unit which pivotally displaces the grate system for access to the hammer roll and grate assembly.
Other devices are aimed at allowing easy access to replace broken or worn components instead of preventing unshredable material from entering the hammer mill the device. For example, U.S. Pat. No. 4,202,503, teaches a hammer mill comprising a housing and mounted within the housing a rotor and a breaker and screening assembly which cooperates with the rotor is constructed so that the breaker and screening assembly may be angularly displaced between an operative position adjacent the rotor and a servicing position at which access may be had to the breaker and screening assembly from outside the casing.
Accordingly, what is lacking in the prior art is a cost effective safety system for a reduction mill that is capable of effectively preventing or minimizing damage caused to the mill components by the introduction of unshredable material. The safety system should achieve objectives such as quick response and reliable performance. The safety system should include packaging flexibility for installation on various new and pre-existing hammer mill configurations including retrofitting onto pre-existing hammer mills with minimal modification.